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    <title>WebGL - Fundamentals</title>
    <link type="text/css" href="resources/webgl-tutorials.css" rel="stylesheet" />
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    <canvas id="c"></canvas>
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<!--
for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
See https://webglfundamentals.org/webgl/lessons/webgl-boilerplate.html
and https://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
for webgl-utils, m3, m4, and webgl-lessons-ui.
-->
<script src="resources/webgl-utils.js"></script>
<script id="vertex-shader-2d" type="notjs">

  // an attribute will receive data from a buffer
  attribute vec4 a_position;

  // all shaders have a main function
  void main() {

    // gl_Position is a special variable a vertex shader
    // is responsible for setting
    gl_Position = a_position;
  }

</script>

<script id="fragment-shader-2d" type="notjs">

  // fragment shaders don't have a default precision so we need
  // to pick one. mediump is a good default
  precision mediump float;

  void main() {
    // gl_FragColor is a special variable a fragment shader
    // is responsible for setting
    gl_FragColor = vec4(1, 0, 0.5, 1); // return redish-purple
  }

</script>
<script>
"use strict";

function main() {
  // Get A WebGL context
  var canvas = document.querySelector("#c");
  var gl = canvas.getContext("webgl");
  if (!gl) {
    return;
  }

  // setup GLSL program
  var program = webglUtils.createProgramFromScripts(gl, ["vertex-shader-2d", "fragment-shader-2d"]);

  // look up where the vertex data needs to go.
  var positionAttributeLocation = gl.getAttribLocation(program, "a_position");

  // Create a buffer and put 12 clip space points in it.
  // 4 rectangles, 2 triangles each, 3 vertices per triangle
  var positionBuffer = gl.createBuffer();

  // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  var positions = [
      -.8,  .8, 0, 1,  // 1st rect 1st triangle
       .8,  .8, 0, 1,
      -.8,  .2, 0, 1,
      -.8,  .2, 0, 1,  // 1st rect 2nd triangle
       .8,  .8, 0, 1,
       .8,  .2, 0, 1,

      -.8, -.2, 0, 1,  // 2nd rect 1st triangle
       .8, -.2, 0, 1,
      -.8, -.8, 0, 1,
      -.8, -.8, 0, 1,  // 2nd rect 2nd triangle
       .8, -.2, 0, 1,
       .8, -.8, 0, 1,
  ];
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

  // code above this line is initialization code.
  // code below this line is rendering code.

  webglUtils.resizeCanvasToDisplaySize(gl.canvas);

  // Tell WebGL how to convert from clip space to pixels
  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  // Clear the canvas
  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Tell it to use our program (pair of shaders)
  gl.useProgram(program);

  // Turn on the attribute
  gl.enableVertexAttribArray(positionAttributeLocation);

  // Bind the position buffer.
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
  var size = 4;          // 4 components per iteration
  var type = gl.FLOAT;   // the data is 32bit floats
  var normalize = false; // don't normalize the data
  var stride = 0;        // 0 = move forward size * sizeof(type) each iteration to get the next position
  var offset = 0;        // start at the beginning of the buffer
  gl.vertexAttribPointer(
      positionAttributeLocation, size, type, normalize, stride, offset);

  // draw
  var primitiveType = gl.TRIANGLES;
  var offset = 0;
  var count = 4 * 3;   // 4 triangles, 3 vertices each
  gl.drawArrays(primitiveType, offset, count);
}

main();
</script>

